1. Field of the Invention
The present invention is generally directed to a material testing machine and more specifically to a latching coupling device for attachment with a material testing system.
2. Description of Related Art
Material testing systems are used in a variety of fields and generally include an interfacer. The interfacer is most commonly a penetrator as used with penetration hardness testers. Penetration hardness testers are well-known in the art, and generally include a diamond or ball tipped penetrator means to apply minor and major loads of predetermined magnitudes through the penetrator to a test specimen in successive load cycles. The hardness is related to the depth of penetration of the penetrator into the surface when a selectable value of compressive force is applied to the penetrator. The testers generate results such as a Rockwell number or Brinell number. Other interfacers commonly used with material testing systems include compression platens, probes, grips, and the like.
Over the years, many material testing systems have been developed. In regards to penetration hardness testers, these systems use a variety of different mechanisms to attach and secure the penetrators to the penetration hardness testers. For example, as shown in FIG. 7, United Calibration Corp., of Huntington Beach, Calif., manufactures machines having a split collet arrangement that accept the ¼″ shank of the diamond penetrator. The penetrator is gripped when the screw is tightened on the provided split collet clamp. The clamp must be tightened while the penetrator is in contact with a specimen under the maximum test load (this is sometimes referred to as “preloading the load string”). Also, the high test loads are borne by the contact between the penetrator shoulder and the end face. Because perfect perpendicularity of the penetrator shank and its shoulder, and the housing bore and its face, is realistically impossible, all four surfaces never have simultaneous contact. This assembly is overconstrained and the parts of the assembly may shift during use which results in inaccurate testing. Settling of the coupling during testing can affect the hardness measurement since penetration hardness testers measure the depth of penetration while applying a penetration load. Furthermore, this assembly is inconvenient because it requires a tool to tighten or loosen the clamping screw, at a specific torque, every time a different penetrator is used to test different materials at various loads. It is not uncommon for the coupling to be changed many times in one day and this routine becomes impractical and leads to unreliable test results.
Other testers, such as those manufactured by Newage Testing Instruments, Southampton, Pa., use a threaded penetrator that screws into the holder until the penetrator shoulder bottoms out. Similar to the United machines, the screw threads must be machined perpendicular to the seating shoulder. Commonly, the penetrators loosen due to everyday wear or because of poor installation. Users of the machine have no clear way of recognizing an alignment problem since the machine provides no clear indication of a loose penetrator. Once the penetrator becomes loose, data is greatly affected. Others, such as Wilson-Wolpert, use magnets in the housing to bring the penetrator into constant, compressive contact with the housing. Yet, this design is flawed because the magnet can attract and retain iron particles from the penetrator or working environment in general. Accordingly, contamination between the mating faces of the penetrator and the housing will affect data.
Another tester, known as the Instron/Wilson tester, made by the assignee of the present invention, uses a spring-loaded-ball assembly threaded into the housing to gently bear on a flat area on the side of the penetrator shank to hold the penetrator in place. This is an improvement over the previously described split collet and threaded designs for a number of reasons. Foremost, the use of a spring provides less overconstraint of the surfaces. Furthermore, with a spring-loaded-ball assembly, there is no need to preload the load string while mounting a penetrator. This type of tester also allows the penetrator to be changed without the use of tools. On the other hand, the spring-loaded-ball assembly of the Instron/Wilson tester can still overconstrain the assembly and lead to non-repeatability. With daily wear and changing the penetrator, the shank and bore can become displaced and non-perpendicular with continued use. Moreover, the spring-loaded-ball assembly is incapable of securing heavier penetrators in place.
Accordingly, what is needed is a latching coupling device that overcomes the problems associated with a typical materials testing assembly. The system should be easily implemented within the existing environment and should be adaptable and compatible with existing technology.